Section 79.—SLIDE AND OTHER VALVE GEAR.
It would be neither easy nor useful, besides being beyond the scope of this work, to attempt to illustrate all the varieties of gear employed to work the valves of steam and other motor engines. I shall therefore only illustrate the more important types in general use, the details of which may be varied to suit individual cases.
[1435]. Is the ordinary slide valve gear with single eccentric for engines running always in one direction.
[1436]. Ordinary link motion reversing gear with two eccentrics; the link, having a shifting motion, is so arranged that either eccentric can be put into gear with the slide valve, the other eccentric running idle; or when in the mid position, as in sketch, both eccentrics run idle and the slide valve has no motion. By setting the link at intermediate positions the travel of the valve can be varied, and consequently the cut off also within certain limits.
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[1437]. Nicholson’s patent reversing gear, without eccentrics. The drawing explains itself. This gear cannot be run in the intermediate positions, as a link motion to vary the cut off. This limits its usefulness to simple reversing only.
[1438]. Automatic governor expansion for single eccentric engine; the position of the connecting rod end in the swinging link is dependent on the governor, and thus also the travel of the slide valve.
[1439]. Side shaft motion for operating Cornish, Corliss, and spindle valves. The valves can be driven from this shaft by cams, eccentrics, or gearing.
[1440]. Gab lever for throwing the eccentric out of gear and thus stopping the engine.
[1441]. Sector and link reversing motion for oscillating engine; sometimes a shifting eccentric is used instead of link motion, as [No. 1443].
[1442]. Reversing sector link motion for an oscillating engine; the valve is operated from the link, the angle of which is altered by the hand lever, there is therefore no lead to the valve.
[1443]. Shifting eccentric and balance sometimes used for reversing instead of double eccentrics and link; the loose eccentric is carried round in either direction by a stop piece on the shaft, fixed so as to give the correct lead both ways.
[1444]. Murdoch’s variable expansion gear (see Mechanical World, September 29th, 1888) has one eccentric which operates a double arm lever, the outer end of which moves a sliding fulcrum along the valve rod lever, so that the leverage of the valve rod lever varies at different parts of the stroke. The sliding fulcrum is attached to a radius rod.
[1445]. Proell’s automatic expansion gear. Shown applied to special double beat valves, but is sometimes applied to a special throttle valve, and is then applicable to any ordinary engine. The action of the governor alters the lap of the catches upon the ends of the valve levers, thus varying the time that the valves are kept open; the catches are centered on an oscillating T lever, operated from an eccentric on the main shaft.
[1446]. Marshall’s valve gear, driven by one eccentric on crank shaft. The sector rocking centre is moved along the curved slot to reverse the engine, giving similar motion to the valve rod as in the case of the ordinary link reversing gear [No. 1436].
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[1447]. The Bremmé valve gear with single eccentric; the valve rod is operated by a lever and bent connecting rod from the end of the eccentric rod; the latter is constrained to move in an arc by a three-link attachment to a fixed bearing behind the eccentric rod and a movable one at the right-hand end of the horizontal link. To reverse, the arm and sector are turned to the dotted position by the worm and hand wheel shaft.
[1448]. Joy’s valve gear, operated by a pin on the connecting rod. The slotted T lever is connected to the hand lever for reversing, and when reversed stands at the same angle from a vertical line but on the opposite side. The fulcrum of the valve rod lever has a sliding motion in the slot of the T lever.
[1449]. Variable expansion gear by hand power. There are many applications of this type used to vary the travel of a cut-off valve.
[1450]. Corliss valve gear, operated by a single eccentric, has two steam and two exhaust valves similar to [No. 1642], worked from pins on a rocking wrist plate. The steam valves have trips, regulated by the governor, on a similar principle to [No. 1445].
[1451]. Crank shaft governor with shifting eccentric: the centrifugal action of the weights, acting against springs, is used to revolve the inner eccentric so as to vary the throw of the main eccentric from which the slide valve is driven.
[1452]. Another form: in this also the throw of the eccentric is varied by the action of the governor ball.
[1453]. Another form of automatic governor expansion trip gear in connection with Cornish valves; a single eccentric operates the four valves, and the contacts of the catches and steam valve levers are regulated by the governor, the lower or exhaust valves having a constant motion.
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[1454]. Double angular slide valve for varying the cut off by a transverse motion given to the valve from outside, either by hand gear or by the governor, the valve being made wider than the valve face, as dotted lines.
[1455]. H. Jack’s variable expansion gear, with one eccentric. Patent No. 4167/85.
[1456]. Variable cut-off valve on the back of the main slide, the rod of which can be revolved by hand or from the governor to vary the opening of the cut off valves.
[1457]. A plan to effect the same object but by a cylindrical cut off valve.
[1458]. English’s expansion gear. Two eccentrics. The expansion valve has no lap, and the gear gives a constant relative motion to both valves.
[1459]. Tappet gear, sometimes used for water-pressure engines, &c.